The field of the present invention relates to data reading systems. In particular, an optical scanning and method for optical scanning are described herein for creating a dense scan pattern. The field of the present invention is especially suitable for use as a fixed scanner such as that employed at a supermarket checkout counter reading bar codes such as those found on consumer products.
For effective and accurate performance, a bar code scanner depends upon focused optics and scanning geometry. Fixed scanners frequently employ a rotating polygon mirror which directs a scanning beam toward a mirror array for generating a desired scan pattern. One type of fixed bar code scanner positions a scan engine in a base with a scan window oriented in a horizontal plane. One such scanning system is disclosed in U.S. Pat. No. 5,073,702 in which a scanning beam is reflected off a mirror array which has a plurality of mirrors arranged in a generally semicircular pattern. The scanning beam reflecting off each of the mirrors has vertically upward component thereby passing through the window/aperture. Objects to be scanned are passed over the window with the bar codes oriented in a generally downward direction.
In another scanner orientation, the scan engine is housed in a vertical tower with the scan window oriented in a vertical plane. In such a vertical scanner, generally all the outgoing scan beams come out sidewards also have an upward vertical component. Objects to be scanned are passed in front of the window with the bar codes oriented in a generally sideward direction.
In order to produce a successful scan, an object must be oriented with its bar code passed in front of the scan window at an angle which is not so oblique as to prevent a scan line from striking or xe2x80x9cseeingxe2x80x9d the bar code. Therefore to achieve a successful scan, the user must position the object with the bar code placed sufficiently close to the desired orientation. The range of suitable plane orientation of the object bearing the bar code is limited by the size of the window and the angle over which the mirror array can direct a scan pattern. Present vertical scanners can scan bar codes oriented on certain lateral sides (i.e. side facing) which face the vertical window, but experience difficulties in scanning faces oriented in a horizontal plane (i.e., facing up or down) or lateral sides opposite the window. Horizontal scanners (i.e. upward facing) are fairly adept at scanning the bottom side but are frequently limited as to which lateral sides may be scanned.
U.S. Pat. No. 5,475,207 (Bobba, Acosta et al.) discloses a variety of single and multiple source laser scanners, and single and multiple scan mechanisms for creating relatively dense scan patterns through multiple scan housing windows. A commercial embodiment of one of the scanners disclosed in the U.S. Pat. No. 5,475,207 patent is the Magellan(copyright) scanner sold by PSC Inc. of Eugene, Oreg.
These scanners generally rely on the number of facets in the polygon mirror and the number of pattern mirrors to generate a certain number of scanning segments or scan lines. The number of pattern mirrors is limited by the size of the scanner housing. The present inventor has undertaken to overcome this limitation as disclosed in Acosta U.S. application Ser. No. 09/360,039 filed Jul. 23, 1999 wherein multiple laser beams are directed along parallel paths onto the polygon mirror for creating additional scan lines.
The present invention is directed to a system for and a method of efficiently generating a dense pattern of scan lines. In a preferred configuration, laser beam is dithered before being directed onto a scanning mechanism such as a polygon mirror which scans the beams across pattern mirrors to generate a higher density scan pattern out into the scan volume. Return light reflecting off an object being scanned is retrodirectionally collected and directed by a collection element such as a collection lens toward a detector. In a preferred construction, the system may optionally include a concentrating element disposed upstream of the detector if needed to compensate for the dithering action by redirecting the return light focused by the collection element back onto the detector.